Forensic DNA analysis of sexual assault evidence often involves analysis of DNA from sperm cells and DNA from other cells such as epithelial cells. The samples obtained from victims often contain a mixture of sperm and other cells such as epithelial cells. Because other cells such as epithelial cells may outnumber sperm cells by many folds, contamination from one or more other sources of DNA may occur while sperm DNA is being extracted. Therefore, it is often desirable to separate the sperm cells and epithelial cells, or separate the sperm DNA from the epithelial DNA as much as possible, prior to analysis. In certain instances, separation and isolation of a particular DNA to create an accurate profile is important for identification of an assailant.
Differential extraction is a broad term used to describe several extraction methods that can be used to separate cells. In certain instances, unique characteristics of sperm cells allow for the differential extraction of the epithelial cells from the sperm cells. One differential extraction procedure was described in 1985 (Gill et al., (1985) Nature 318: 557-9). In certain instances, separation of the sperm cell fraction from the victim's DNA profile decreases ambiguity in the results and allows for easier interpretation of the perpetrator's DNA profile in a rape case.